The present invention relates to a line state detecting apparatus for detecting a state of a transmission line in a balanced transmission system for transmitting data by using a pair of conductors, and a transmitting apparatus and a receiving apparatus of a balanced transmission system provided therewith.
In a background art, there is widely used a balanced transmission system for transmitting data by using a pair of transmission lines brought into a balanced state. In a balanced transmission system of this kind, as in, for example, a telephone line, a transmission line by an exclusive communication line or the like is frequently used, normally, some degree of balance is moderately maintained by current flowing via a forward line or a rearward line of pair of transmission line.
FIG. 31 shows an example of a transmitting portion and a transmission line of a balanced transmission apparatus of a background art. The balanced transmission apparatus is constructed by a constitution of transmitting a signal to transmission lines comprising a pair of conductors W1, W2 from a transmitting portion 92 via a transmission transformer T91. In this case, a current of a transmission signal transmitted from the transmitting portion 92 depends of characteristics of the transmitting portion 92 and the transmission lines W1, W2.
Although the transmission lines reaching a receiving portion from the transmitting portion are inherently requested to be completely balanced, actually, there are unbalance elements in the transmitting portion and the receiving portion, unbalance components by a situation of wires of the transmission lines and an apparatus connected to a middle of the transmission lines and the like, and it is an actual situation that currents flowing in the conductors W1, W2 of the transmission lines are not regarded to be completely balanced. When the currents flowing in the transmission lines are unbalanced in this way, a portion of a transmission power leaks from the transmission line to outside to deteriorate a transmission characteristic of data or bring about a drawback of interference or the like.
Recently, there is proposed a balanced transmission system for transmitting data by superposing a high frequency signal on a power line for carrying a power of a commercial power source or the like. Particularly, such a power line is not inherently a line for communication and therefore, balance may be dispersed by an individual household wiring situation, an apparatus connected to a power source or the like. Therefore, when the power line is used as a transmission line, there is a concern that the balance is changed by individual environments and a transmission characteristic significantly differs.
Therefore, although it is preferable to promote of balance of transmission lines by detecting a state of the transmission lines and controlling currents or voltages in two lines in a balanced transmission system, in a background art, detection and control of such a line state have not been executed.